reflexes
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KINE 3301 Biomechanics of Human Movement. Reflexes. Chapter 19. Monosynaptic & Polysynapic Connections. The Ia afferent neuron from the muscle spindle is the only sensor fiber to make a monosynaptic connection. . Presynaptic Inhibition & Facilitation. - PowerPoint PPT PresentationTRANSCRIPT
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Reflexes
Chapter 19
KINE 3301Biomechanics of Human Movement
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Monosynaptic & Polysynapic Connections
The Ia afferent neuron from the muscle spindle is the only sensor fiber to make a monosynaptic connection.
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• The efferent neural response can be modified by monosynaptic or interneuron input.
Presynaptic Inhibition & Facilitation
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Muscle Spindle Structure
Intrafusual Fibers: the term intrafusal fibers refers to the muscle spindle to differentiate it from the extrafusal (normal muscle). Fusimotor System: the fusimotor system refers to the muscle spindle, it’s afferent, efferent and specialized muscle tissue.
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Nerve Conduction velocity:
Ia 40 – 90 m/sII 30 – 75 m/s
Spinal Connections
The Ia neuron makes monosynaptic connections with alpha and gamma.
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When a muscle spindle is stretched above it’s current sensitivity it elicits a stretch reflex:1. Ia stimulates alpha –
agonist & synergist.2. Ia inhibits antagonist.3. Ia stimulates gamma
Stretch Reflex
The number of impulses propagated up the Ia neuron is related to the magnitude and rate of stretch.
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Inverse Stretch Reflex
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The Ib neuron weaves in and out of the tendons collagen fibers. The GTO is very sensitive to changes in muscle force. It can respond to a twitch from a single motor unit.
Note it uses interneurons to communicate with alpha.
Golgi Tendon Organ (GTO)
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A motor unit is one alpha motor neuron and all of the muscle fibers it innervates.
GTO & Muscle Spindle Control Muscle Stiffness
𝑀𝑢𝑠𝑐𝑙𝑒𝑆𝑡𝑖𝑓𝑓𝑛𝑒𝑠𝑠= ∆𝐹𝑜𝑟𝑐𝑒∆ h𝐿𝑒𝑛𝑔𝑡
𝑀𝑢𝑠𝑐𝑙𝑒𝑆𝑡𝑖𝑓𝑓𝑛𝑒𝑠𝑠=𝐺𝑜𝑙𝑔𝑖𝑇𝑒𝑛𝑑𝑜𝑛𝑂𝑟𝑔𝑎𝑛𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑒𝑠∆𝐹𝑜𝑟𝑐𝑒𝑀𝑢𝑠𝑐𝑙𝑒𝑆𝑝𝑖𝑛𝑑𝑙𝑒𝑟𝑒𝑔𝑢𝑙𝑎𝑡𝑒𝑠∆ h𝐿𝑒𝑛𝑔𝑡
The level of muscular preactivation prior to some known event is modulated by the golgi tendon organ and the muscle spindle via cortical commands.
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Reflex latencies from cutaneous receptors vary from 50 – 85 ms.
Cutaneous Receptors
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Convergence of signals to the motor neuron can modulate the input to the motor neuron and it’s output response to a stimulus.
Convergence & Modulation of Sensory Signals
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• Reflexes were once thought to be invariant. It was assumed that they were completely involuntary.
• We now view the terms reflex and voluntary as being on a continuum with pure reflexes such as an eye blink or a stretch reflex on one end and complete and full conscious control on the other end.
• The speed with which a movement or force is generated determines the extent to which feedback is able to correct errors.
• In feedback control the movement is slow enough (> 50 ms) that reflex signals can modify movement.
• In feedforward control the movement is execute so fast (< 50 ms) so that feedback can not modify movement.
Feedback and Feedforward Control
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• The minimum time for the neuromuscular system to respond to a stimulus is 80 – 200 ms.• Reflex time 30 – 120 ms• Muscle force time 50 – 300 ms
Consider the following scenarios, which are feedback and feedforward:• During the upward phase of a vertical jump can you
change from jumping up to jumping up & forward?• When landing on a rug, if the rug is unexpectedly
pulled backward at the instant your feet touch the rug, what happens?
• 10 ms prior to a bat hitting a ball can you switch from hitting a fly to hitting a grounder?
Feedback & Feedforward